Pfizer hits accelerator on Xtandi trial in early prostate cancer

US pharma giant expects PROSPER trial results later this year

Pfizer and partner Astellas have brought the deadline for a trial of their Xtandi drug in earlier-stage prostate cancer forward by almost two years as they fight for market share with Johnson & Johnson's rival Zytiga therapy.

Originally due to report in mid-2019, the large-scale PROSPER trial of Xtandi (enzalutamide) in patients with non-metastatic, castration-resistant prostate cancer will now generate top-line results later this year.

Pfizer and Astellas have tweaked the protocol so the sample size has been reduced to 1,440 patients from 1,560, but the main outcome measure - metastasis-free survival - is unchanged.

The decision comes shortly after J&J reported the results of the LATITUDE study of Zytiga (abiraterone acetate) in men with newly-diagnosed prostate cancer at the ASCO meeting in Chicago, revealing a 38% reduction in the risk of death compared to placebo that was described by its investigators as "practice-changing".

That trial was backed up by another study - called STAMPEDE - which showed that adding Zytiga to newly-diagnosed, low-risk prostate cancer patients bolstered survival and reduced the chances of treatment failure.

Both studies have been billed as having the potential to drive uptake of J&J's drug, adding pressure on Xtandi. And Pfizer and Astellas' position was further weakened by another trial called PLATO, which showed that adding Xtandi to Zytiga therapy conferred no additional benefit.

Faced with being pushed back in the treatment sequence for metastatic prostate cancer, PROSPER could be massively important for Xtandi thanks to improved diagnosis showing that less than 20% of men have imaging evidence of metastasis at first referral, according to a report in the journal Oncology.

Getting approval for earlier-stage disease - before the cancer has spread - could extend the reach of Xtandi as well as insulate the drug from competition.

Xtandi and Zytiga have both been instrumental in improving treatment for metastatic prostate cancer in recent years, but both brands could be affected when generic versions of J&J's drug start to appear in the big US market within the next 18 months.

Xtandi was first approved in the US in 2012 for second-line metastatic CRPC treatment after docetaxel-based chemotherapy, and claimed a first-line indication two years later when Zytiga sales were already running at more than $2bn.

Since then Pfizer and Astellas' drug has grown strongly, in part because it can be given on its own while Zytiga needs to be given with a corticosteroid (prednisone) to counter side effects, and Zytiga's sales have started to taper off reaching $2.26bn last year, which was fractionally behind Xtandi's $2.3bn turnover.

Analysts have suggested that with the new data Zytiga could see a resurgence, potentially pushing sales above the $3bn mark.

Pfizer and Astellas suffered another setback recently with Xtandi, after they decided to terminate the ENDEAR trial of the drug in triple-negative breast cancer, and shelve plans to pursue that indication.